1. Field of the Invention
The present invention relates to a coaxial filter for use with a transmission line. More specifically, the present invention relates to a coaxial filter, comprising a metallic casing coupled to a transmission line, wherein a dielectric resonator is provided.
2. Description of the Prior Art
A coaxial filter is often required in various types of transmission lines such as a waveguide, coaxial cable, strip line and the like. To that end, the inventors first implemented a coaxial filter of interest to the present invention as shown in FIGS. 1A, 1B and 1C, wherein FIG. 1A shows a plan view of a band rejection coaxial filter for use with a coaxial transmission line with the top plate removed to show the internal structure thereof, FIG. 1B shows a perspective view of a dielectric resonator for use in the FIG. 1A filter and FIG. 1C shows a sectional side view of a portion including a dielectric resonator of the FIG. 1A filter. The filter shown comprises a metallic casing 1 made of aluminum, brass, copper or the like, which is formed of an input opening, not shown, at the left side plate, as viewed, and an output opening, not shown, at the right side plate, as viewed. An input coaxial line 2 is coupled to the input opening by means of an input connector 21 and an output coaxial line 3 is coupled to the output opening by means of an output connector 31. An inner conductor 4 is provided to span the input and output connectors 21 and 30 such that the inner conductor, not shown, of the input coaxial line 2 and the inner conductor, not shown, of the output coaxial line 3 are electrically connected while a characteristic impedance is matched with respect to the casing 1. As a result, the inner conductor of the input coaxial line is connected to the inner conductor of the output coaxial cable 3 through the inner conductor 4.
Three dielectric resonators 5, for example, are disposed in the casing 1 in the vicinity of the inner conductor 4, spaced apart a distance d from the inner conductor 4 and spaced apart from each other a distance commensurate with a quarter wave length or .tau./4 of the resonance frequency of the resonators. It is pointed out that the internal geometry of the casing 1 is selected such that the casing 1 exhibits a cut off state with respect to the frequency band to be used. Accordingly, a transmission signal of UHF or SHF transmitted from the input coaxial line 2 is propagated along the inner conductor 4 to the output coaxial line 3 while a portion of the signal of the frequency determined by the above described dielectric resonators 5 is rejected. More specifically, the filter shown constitutes a band rejection filter, wherein a portion of the signal of the frequency determined by the characteristic of the dielectric resonators 5 is trapped by the dielectric resonators 5, whereby the same is prevented from being withdrawn to the output coaxial line 3.
According to such previously implemented coaxial filter as shown in FIGS. 1A, 1B and 1C, it is necessary that the distance from the dielectric resonator 5 to the upper and lower plates 12 and 11 of the casing 1 be sufficiently large enough to avoid any adverse affect, which necessitates a supporting member such as a spacer 6 to support the dielectric resonators 5 in the central position of the inner conductor 4 in the casing 1. Accordingly, the distance H between the upper and lower plates 12 and 11 of the casing and the other geometry are relatively large. Therefore, the inner conductor 4 also must be of a large diameter to achieve an impedance matching with respect to the casing 1. Because of an increased size of the inner conductor 4, some reinforcing means is required to maintain the mechanical strength of the inner conductor 4, with the result that the structure becomes complicated. In addition, a change of the geometry of the casing 1 results in mismatching of the impedance of the inner conductor 4 and accordingly requires readjustment of the impedance matching of the inner conductor 4. Thus, difficulty and complicacy are encountered in actual use of the previously implemented filter.